1. Field of the Invention
The present invention relates to apparatus for testing gas sensors and, more particularly, apparatus for testing carbon monoxide gas sensors.
2. Description of the Prior Art
Health and safety hazards that are associated with exposure to carbon monoxide and other gases have led to the development of various types of gas detectors. Such devices are based on various technologies including infrared absorbance, electrochemical oxidation, metal oxide conductivity, and chemical color changes. The different technologies offer various advantages and disadvantages, but each generally requires periodic calibration. Even for those instruments in which calibration is not required, verification of the instrument""s functionality is usually preferred. However, such calibration has not always been available.
Various calibration and verification tests have been used in the prior art. The exact mechanism of these tests depends on the sensor technology. Some testers were based on electrical property measurements such as electrical resistance or electrode capacitance as described in U.S. Pat. No. 5,202,637. Other tests are based on measurement of electrochemical properties. However, such prior art testers were subject to various difficulties and disadvantages. For example, such tests tended to provide a simple positive or negative resultxe2x80x94if the result is negative the sensor is deemed to be non-functional. However, such prior art tests were designed to assess only selected failure modes. If the sensor were to fail in a non-design mode, the result would be a false-positive indication. For example, an electrochemical sensor would lose sensitivity and yield false-positive results to an electrical resistance or electrode capacitance test if the diffusion limiting membrane were blocked by foreign material. As another example, a number of sensors use catalysts that can be deactivated by poisoning agents. However, electrical tests of the sensor would not detect this fault.
Other sensor testers such as described in U.S. Pat. Nos. 4,151,739 and 4,267,030 relied on an electrochemical electrolysis cell that was incorporated into or near the sensor. The sensor was generally responsive to the gas that the electrolysis cell generated, but the test process was indirect because the generated gas was not the gas of interest. For example, one electrolysis cell described in European Patent Application EP0744620A1 produced hydrogen gas through the electrolysis of sulfuric acid. However, it was recognized that if the gas of interest was other than hydrogen, as, for example, carbon monoxide, it would be preferable to test the sensor by direct exposure to the gas of interest.
A more reliable test methodology is to expose the sensor to the target gas and monitor the sensor response. However, prior art methods generally required the use of heavy or bulky compressed gas cylinders or complex electrochemical reactions to generate the target gas. For example, carbon monoxide sensors in industrial applications are frequently exposed to carbon monoxide sourced from a gas cylinder. Moreover, convenient sources of the target gas are not always available. Thus, in some applications, as in residential usage, the carbon monoxide sensors frequently are never field calibrated at all.
Accordingly, there was a need in the prior art for a gas sensor tester that would expose sensors to the target gas and monitor the reaction of the sensor. Additionally, there was a need for a sensor tester that was relatively compact and light, that would work with various types of sensors, and that provided a reliable, repeatable test environment.
In accordance with the subject invention, apparatus for testing a gas sensor includes an array of electrical resistors that are attached to a base member. One end of each resistor is electrically connected to a common terminal and each resistor is coated with a compound that emits a predetermined gas in response to increases in temperature of the resistor.
Preferably, the testing apparatus further includes means for generating an electrical voltage. Also preferably, the coating compound includes a metal oxalate wherein the metal is selected from the group consisting of alkali metals and alkaline earth metals.
More preferably, the voltage generating means of the testing apparatus includes means for generating a voltage at a predetermined time in combination with an electrical network that is connected to the voltage generating means and to the array of electrical resistors, wherein the network selectively connects individual resistors of the array to the voltage generating means. Also, more preferably, the coating on the resistors is a compound that includes an oxalate salt of a metal, a binder, and a thickener.
Most preferably, the apparatus for testing the gas sensor is included in the gas diffusion path of a sensor of the type wherein electrolyte is maintained in the body of the sensor and a plurality of electrodes are in contact with the electrolyte. Also most preferably, the metal of the metal oxalate is selected from the group of sodium, lithium, calcium, potassium and magnesium.
Other objects and advantages of the invention disclosed herein will become apparent to those skilled in the art as a description of a preferred embodiment of the invention proceeds.